1. Field of the Invention
The present invention relates to the preparation of microporous films. More particularly, the present invention relates to an improved "solvent stretch" process and a microporous film product having an improved morphological structure.
2. Summary of the Prior Art
A process for preparing microporous films is disclosed in U.S. Pat. No. 3,839,516 which is herein incorporated by reference. This process, hereinafter referred to as the "solvent stretch" process, includes the consecutive steps of (1) contacting a polymeric precursor film having at least two components, one of which is lesser in volume than all the other components (e.g., an amorphous component and a crystalline component), with a swelling agent for sufficient time to permit absorption of the swelling agent into the component of lesser volume, (2) then stretching the precursor film in at least one direction while in contact with the swelling agent, and (3) maintaining said film in its stretched state during removal of the swelling agent. The microporous films themselves are optionally stabilized by heat setting or ionizing radiation. The precursor films from which the microporous films are prepared in accordance with this process are characterized as having a percent of crystallinity of at least 30 percent, preferably at least 40 percent, and most preferably at least 50 percent (e.g., 60 to 90% or higher) and a film thickness of about 0.1 to about 250 mils or thicker.
At one time it has been preferred to employ a particular starting polyethylene resin, Fortiflex A-60-500 resin manufactured by Soltex Polymer Corp., for use in the "solvent stretch" process since it had been found that this particular resin yielded microporous films having enhanced permeability and porosity. However, the reasons that this resin functioned in the "solvent stretch" process better than certain other resins, even certain other polyethylene resins, were not understood. Since that time, the A-60-500 resin has been discontinued for production and is no longer available on the market. Consequently, a search was instituted to uncover a starting resin which could match or better the performance of the discontinued resin. This search was complicated by the lack of understanding by those skilled in the art as to what particular polymer properties were most influential in improving and controlling the properties of a microporous film prepared therefrom.
The above-described patent which discloses the basic "solvent stretch" process contains a broad list of possible starting polymers for the production of microporous films.
For example, the polymers which may be utilized to prepare the precursor film for use in the solvent stretch process are disclosed as including olefinic homopolymers, copolymers, as well as numerous other types of polymers. These polymers such as, for example, polypropylene, are further characterized as possessing a broad melt index of from about 0.1 to about 75, and preferably from about 0.5 to 30, and a weight average molecular weight ranging from about 50,000 to 750,000 and preferably from about 200,000 to 500,000. This patent neither specifically characterizes molecular weight distribution ratio (M.sub.w /M.sub.n) nor density as criteria for selecting a particular polymer from the broadly disclosed classes of polymers. Example 1 thereof does disclose that the polyethylene resin has a particular M.sub.w and a particular M.sub.n (from which the molecular weight distribution ratio may be calculated to be 14.5).
It is also known to utilize a resin having a high melt index and low molecular weight to provide a precursor film utilized in a "dry stretch" process to prepare a microporous film as evidenced by U.S. Pat. No. 3,839,240, and it is further known to incorporate organic and inorganic fillers into porous films prepared by a variety of methods as evidenced by U.S. Pat. Nos. 3,903,324; 3,862,030; and 3,738,904. However, such methods do not employ a "solvent stretch" process and are therefore not directed toward solving the particular problems associated therewith.
There has been a continuing search for methods to improve the morphological structure of microporous films prepared by the "solvent stretch" process.
It is therefore an object of the present invention to provide an improved "solvent stretch" process for preparing a microporous film having an improved morphological structure.
These and other objects, as well as the scope, nature and utilization of the present invention will be apparent to those skilled in the art from the following detailed description and appended claims.